Photocrosslinkable polymers are known. They have found many applications in industry. For example, photocrosslinkable polymers find many applications in the making of lithographic plates and in photoresist applications. Many of the more useful photocrosslinkable polymers comprise, as a light-sensitive component, recurring cinnamate components. For example, polyvinyl cinnamate is formed by reacting polyvinyl alcohol with cinnamic acid. Such polymers are disclosed in U.S. Pat. No. 3,817,876, U.S. Pat. No. 4,230,817 and U.S. Pat. No. 3,933,746.
In photolithographic and in photoresist applications, photocrosslinkable polymeric compositions are coated upon a suitable supporting surface such as a metal plate to form an imaging element. Thereafter, the element is exposed to actinic light. Exposure results in insolubilization of the polymer in the exposed areas and solubilization of the polymer in the unexposed areas. The unexposed areas of the polymer coating can be removed by means of suitable solvents to leave a relief image.
The photosensitized crosslinking of amorphous polymers such as those described in the above-mentioned U.S. patents generally show quantum yields of dimerization of about 4 percent. Sensitization of such polymers by ketocoumarins proceeds generally by triplet-triplet energy transfer. There are known to be several energy wastage steps competing with triplet-triplet energy transfer. In the case of amorphous cinnamate polymers, the triplet energy migrates step-wise from one cinnamate component to the next until it decays, or reaches an appropriate site for promoting crosslinking. An appropriate site in this regard is a double bond of the cinnamate components aligned parallel to each other, and approximately 4 angstroms apart. The triplet energy visits an average of 100 to 200 sites before an appropriate site is found. Since the lifetime of the triplet excited state is only 10 nanoseconds, the decay to the ground state dominates over excimer formation which results in crosslinking.
It would be extremely important and useful to design polymers which would make better use of triplet-triplet energy transfer which occurs with ketocoumarin sensitization.